Jan 10th, 2013

Since Sirius is more than 8 light-years away, you’re not seeing it as it is right now, but you’re seeing what it looked like in 2005 when all you could do with most mobile phones was talk to people and not stream your favorite television program. It’s taken that long for the light from Sirius, a star over twice as large as our sun, to reach our eyes on planet Earth.

Sirius is the brightest star visible to the unaided eye, from any point on Earth’s surface. So you really shouldn’t have any problem spotting it. But in case you do, just find the three slanting Belt Stars, in the Orion constellation. Follow the downward slope, to the left, and keep moving out-until you land at “Sothis.”

That’s what the Ancient Egyptians called Sirius. Many sub-Saharan cultures also have names, for this ancient celestial clock. The Serer of Senegal, in their cosmology, identify Sirius as “Yoonir,” while in Kenya and Ethiopia, the Borana name for the object is “Obora Dikka.”

By whatever name, this hot and very luminous star is familiar to almost everyone on planet Earth. Visible from most vantage points, its influence on human culture, especially that of Africa, is pervasive and pronounced.

Sirius is described variously as “bluish white” or “white.” Colour aside, it radiates 20 to 25 times more visible light than does the Sun.

In fact, Sirius has an apparent magnitude of -1.46—one of just four visible stars with negative figures. (The brighter an object appears, the lower its magnitude number.)

“Of the 21 brightest stars,” writes Joe Rao, in Space.Com, “twelve are considered to be of first magnitude, and five are of zero magnitude. But only Arcturus, Alpha Centauri, Canopus and Sirius are bright enough to be assigned a magnitude value lower than zero…”

The diameter of Sirius is 1.7 times that of the solar disc; and it has more than twice the Sun’s mass. But the visual dominance of the star arises, not only from these intrinsic properties but also from close proximity. At 8.6 light years, Sirius is one of the four or five nearest stellar objects to the Sun.

Among stars, only the Sun has been more extensively studied. In 1862, a second star was seen revolving about Sirius, with a period of roughly 50 years. “Sirius-B,” as it is called, has 98 per cent of the Sun’s mass. But is squeezed into a volume of space just 12,000 km in diameter—about the size of Earth.

Since “B” is 10,000 times dimmer than its primary, astronomers calculate that it must consist of very dense matter-so dense, that two or more tonnes could fit in a teaspoon! The Sirius system, therefore, contains what NASA insists is “the first white dwarf star discovered.”